The present invention relates to mass spectrometers, and more particularly, to utilizing mass spectrometers for endpoint detection during the etching steps of semiconductor fabrication.
During the fabrication of semiconductor devices, many layers of the device are etched utilizing plasma etching techniques. Often, the various steps within a plasma etch are ended by detecting a change within the plasma or a change in the gas phase species produced by the reaction of the plasma with the wafer being etched. Such an approach for ending a step within a plasma etch is known as endpoint detection. One common technique for detecting an endpoint for a plasma etch is to monitor the optical emissions of the plasma. However, such system do not adequately sense endpoints in all environments, especially in downstream etching techniques. Downstream etching is a method in which the substrate to be etched is not directly within the RF plasma, but rather, downstream of the plasma. Optical emission endpoint detection systems generally do not provide an adequate sensitivity for use with downstream etching in a production environment.
An alternative approach for endpoint detection is to utilize a mass spectrometer. In particular, a quadrupole mass spectrometer may be utilized. In such an approach, the mass spectrometer may be mounted to the etch apparatus to provide access to either the plasma process chamber or the downstream exhaust from the plasma process chamber. FIG. 1 shows a side view of a schematic of a typical electron impact ionization mass spectrometer apparatus 100 as may be utilized for endpoint detection. The mass spectrometer apparatus 100 may include a flange 105 for connecting the apparatus 100 to the process chamber or process exhaust line of the etch apparatus. The mass spectrometer hardware is located within the apparatus 100. The mass spectrometer hardware includes a filament 115, a focusing lens 125, an ionizer grid 120, a mass filter 130, and a detector 135. The filament 115 ionizes molecules. Electrons are accelerated from the filament 115 to the impact ionizer grid 120 by a voltage which is applied between the filament and the grid. A focusing lens 125 focuses ions into the quadrupole mass filter 130. The focusing lens 125 may include multiple lenses. The quadrupole mass filter 130 has a RF signal applied to four rods to select a desired mass to charge ratio of ions that pass through the filter 130 to be detected on a detector 135. The detector 135 may be either an electron multiplier or a Faraday cup. The mass spectrometer hardware may be mounted within a housing 101 on an end mounting plate 140. Because the filament 115 must be operated at low pressures, typically 10.sup.-4 Torr or less, a differential pump 150 is required to lower the pressure within the ionization chamber 155 formed by the housing 101. The mass spectrometer hardware described above is well known and is commercially available from several sources including the Micromass model from VG, the Dataquad model from Spectramass, and the model 100C from UTI.
Utilizing a standard mass spectrometer system as described above presents several problems. First, the life time of filament 115 is short and unpredictable. Thus, the filament would have to be changed often for use in a production endpoint detection system. Moreover, changing the filament would require accessing the chamber formed by housing 101. Therefore, the maintenance downtime to replace the filament is greatly increased due to standard venting, cleaning and pump down techniques. Thus, it would be desirable to provide an endpoint detection system which minimizes the problems discussed above.